1. Field of the Invention
The present invention relates to a process for the production of ultra-pure silicon bars, and, more especially, to the production of ultra-pure silicon bars by pyrolysis of monosilane on extended silicon supports heated to redness, the product silicon bars being well adapted for use in the semiconductor electronics industry.
2. Description of the Prior Art
It is known to this art to produce ultra-pure silicon bars useful in the semiconductor industry by reduction with hydrogen of a gaseous halogenated silicon compound, such as silicon tetrachloride or trichlorosilane, and by depositing silicon of high purity onto red-heated supports made of silicon or a metal having a high melting point, such as tungsten. Such processes have been described, for example, in U.S. Pat. Nos. 3,023,087, 4,173,944, 4,179,530 and 4,311,545.
However, the decomposition by pyrolysis of monosilane onto a support heated to redness, to produce silicon bars of electronic purity, differs from the reaction which commences from a halogenated silicon compound in respect of, in particular, the starting materials, the very different by-products formed and the temperatures of the gases and the bars, which are also very different. A device for decomposing monosilane has also been proposed to this art, according to U.S. Pat. No. 3,147,141. The process carried out in this device does not enable useful rates of decomposition to be obtained for large bars under economically acceptable conditions, and furthermore it gives rise to high energy costs and requires the use of a device for absorbing hydrogen.
To reduce the effects of these disadvantages, a process and a device for decomposing pure silane to obtain silicon bars have also been proposed to this art, according to U.S. Pat. Nos. 4,147,814 and 4,150,168. According to this process, pure, that is to say, undiluted, monosilane is introduced into the decomposition bell by injectors arranged at several points along the decomposer. Furthermore, in order to obtain a bar having the most regular shape possible despite the substantial increase in the heat emitted by the bars as they grow in diameter, the decomposer contains internal heat shields cooled by water circulation and situated between the different bars.
Such a process for the deposition of silicon has, however, the following disadvantages:
(i) as a result, in particular, of the gas phase decomposition of the monosilane to a powder which is detrimental to the satisfactory operation of the decomposer, the material yield of the bell, namely, the productivity in silicon deposited relative to the silicon introduced in the form of monosilane, proves to be unsatisfactory;
(ii) furthermore, the rate of deposition of the silicon, and also the diameter which can be attained by the bar when deposition is complete, are inadequate.
In relation to the processes and devices/apparatus of the prior art, serious need continues to exist for process/ apparatus satisfying or providing for the following objectives:
(i) to increase the rate of deposition of the silicon, which results in a lowering of the consumption of electricity and an increase in productivity in silicon deposited;
(ii) to increase the final diameter of the bar obtained; and
(iii) to reduce the amount of by-products, namely, to increase the material yield of the decomposer.